How to force creases into a cloth object?
You should probably use pinning for this. If I understand correctly what you want vertex groups will allow you to define parts of the mesh not altered by the cloth simulation.

Select a few corner vertex, as many as you need, the more selected vertex the stiffer the animation will be, and assign them to a new Vertex Group.



Use that vertex group for pinning under the Physics > Cloth > Pinning.
You can use this in conjunction with shape keys and animation to drive the shape from flat to folder or similar.